ALBANY — Gov.-elect Andrew Cuomo won’t take office for two weeks, but two of the state’s most powerful public-employee unions yesterday fired a salvo against the Democrat’s marquee proposal to cap property taxes.

The labor-backed advocacy group New Yorkers for Fiscal Fairness launched a blistering broadside against Cuomo’s bid to limit local property-tax increases at 2 percent annually, dismissing the proposal as “a coward’s way” to address the soaring levies.

“Tax caps make a nice sound bite, but they are not sound policy,” said Ron Deutsch, the group’s director.

“The Cuomo cap would institutionalize the current inequities in school funding . . . and do little more than guarantee that property taxes will continue to rise.

“It’s as if he is creating the perfect storm to decimate the services that so many New Yorkers rely on,” Deutsch said.

The group, whose sponsors include New York State United Teachers, 1199 SEIU and the Civil Service Employees Association, backs a $1.3 billion alternative relief plan paid for, in part, by renewing a soon-to-expire tax hike on the rich.

The attack provides the latest evidence that getting the cap passed could be Cuomo’s first big political fight. It follows complaints by mayors and county executives who said any proposal should include efforts to cut state-mandated government costs.

Kenneth Adams, president of the Business Council of New York, said failing to pass a cap would be “a disaster for New York’s economy.”

“Groups tied to the teachers and public-employee unions are calling for a ‘circuit breaker’ rather than a tax cap,” he said. “That would simply shift New York’s unbearable property-tax burden from some taxpayers to others. It’s a gimmick.”

Three quarters of the state’s voters support Cuomo’s plan, according to a Siena poll released Monday. The cap would have no impact on New York City.

Individually, groups like NYSUT have adopted a friendlier approach, pledging to “work with” Cuomo to find a compromise.

A Cuomo spokesman called the cap a “cornerstone” to the governor-elect’s agenda.

“New Yorkers’ property taxes cannot continue to increase at the same astronomical rate they have over the past decade,” said the spokesman, Josh Vlasto. “New Yorkers can no longer afford it, and families are being driven out of their homes and out of the state.”